I just watched this video, and I'm a bit perplexed.


The radius of Circle A is 1/3 the radius of Circle B.

Circle A rolls around Circle B one trip back to its starting point.

How many times will Circle A revolve in total?

The intuitive answer is 3, but the correct answer is 4. I understand the trick -- that the center of Circle A must travel a distance of $2\pi(r_B + r_A)$, not $2\pi r_B$ -- but I'm still confused on one item.

At the risk of sounding very un-mathematical, how do the (infinite set of) points on the circumference of each circle map to each other to accomplish this?

Consider Circle A rolling along a straight line the length of the circumference of Circle B. Then it will revolve 3 times. It's like the universe "knows" when to apply a different point mapping when you change the arrangement of matter.


Thank you everyone for your answers. They were all informative, but I wasn't able to intuitively understand them until I saw this visual:


The key for me was to see that there are two kinds of revolution happening:

  1. Revolutions of Circle A with respect to Circle B, and
  2. Revolutions of Circle A with respect to the (overhead) observer.

To the observer, the revolutions of the type (2) complete before revolutions of type (1). The revolutions of type (1) happen at the $r_B/r_A$ roots of unity, and the revolutions of type (2) happen at the $(r_B/r_A + 1)$ roots of unity. It was very helpful to see precisely where these revolutions occur, because it was mentally impossible to unify the two types of revolution without the visual. It's also useful to notice that at any given moment, with respect to the observer, the points of Circle A on the far side from Circle B are moving faster than the points of Circle A on the side that is touching Circle B.

David K's answer is great for understanding that the parametric mapping of time to point pairs is the same whether Circle A is rolling along a straight line or a circle, and that we are dealing with frames of reference. I simply didn't believe the mapping was the same until I saw the visual.

zoli and Hans Lundmark's answers are great for understanding that one extra revolution must occur at some point along the rolling path. The complete answer, of course, is that this last mysterious revolution does not happen all at once, but gradually along the entire roll.

The visual was discovered in the comments on this page, which is another discussion of the same problem:


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  • $\begingroup$ This is a very nice summary with good links. Thanks! $\endgroup$ – David K Jul 10 '15 at 12:36

As it is depicted on the figure below the smaller circle has to turn around $3$ times if it travels along a straight distance equaling the perimeter of the larger circle. In this case the center of the smaller circle takes the same distance.

Now imagine that the straight line of length $6\pi r_B$ takes a complete rotation around its center while the small circle rolls along. Also, suppose that the straight takes one full turn while the circle reaches the other end.

As a result the small circle takes a fourth revolution.

The number of rotations is not different if the path is circular. The circularity of the path was modeled in the case of the straight path by turning around the segment.

The video explains the same by claiming that the small circle has to travel a distance of $8\pi r_B$. This is the same as having to travel on the straight line of length $8\pi r_B$ which does not rotate about its center.

enter image description here

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  • $\begingroup$ Thank you for answering, but your answer is basically a re-hash of the video. Like I said, I understand the trick -- that the center must travel $2π(r+r/3)$ -- which dictates that the answer must be 4. I'm trying to find a deeper understanding for why this must be so -- an explanation for why the infinite point mapping of Circle A's circumference to a line changes when that line is straight rather than circular. $\endgroup$ – Ben Simmons Jul 6 '15 at 7:35
  • $\begingroup$ I will add something to my answer for your enlightment. $\endgroup$ – zoli Jul 6 '15 at 7:38

The mapping of points from one circle to the other is no different than when the small circle is rolling on a straight line; the extra revolution is purely due to bending.

It might be easier to see this in the case of gliding, when there is a single point $A$ on the small circle which is in contact with the line or the large circle throughout the process. When gliding on a line, the small circle doesn't revolve at all. But if it's supposed to glide along a large circle in such a way that the same point $A$ is in contact all the time, then it also has to revolve as at glides (and it will make one revolution as it glides one lap along the large circle).

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enter image description here

Circle $A$, with radius $r$, gets back to its starting point when $A$'s centre completes one rotation (around the centre of circle $B$ with radius $R$). Clearly $A$'s centre traverses a circular path of radius = $R+r$.

Now, Physics to the rescue.

Let the speed of $A$'s centre = $v$

Let the angular speed of $A$'s rotation around its own centre = $\omega$

Since, $A$ rolls over $B$ $\implies$ $v = \omega r$ (Assuming $B$ is fixed, the condition of rolling is that the point of contact is at rest).

Let the time taken by $A$'s centre to complete one rotation be $t$.

Then, $2\pi (R+r) = vt$

$\implies t = \frac{2\pi (R+r)}{v}$

Total angular distance traversed by $A$ around its centre in the same duration: $\theta = \omega t$

Using the above results, we get $\theta = \frac{v}{r} \frac{2\pi (R+r)}{v} = \frac{2\pi (R+r)}{r}$

In this time $t$, $A$ completes, say, $N$ rotations around its centre.

$\implies N = \frac{\theta}{2\pi} = \frac{(R+r)}{r}$

Now, In your case $ r = \frac{R}{3}$

Using this information, $ N = 4$

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I don't know what you mean by "the points map to each other". To "map" one set to another implies a function that maps each point in one set to a unique point in the other; to "map to each other" implies that the function is one-to-one. There is no one-to-one mapping between points: each point on the small circle meets three different points on the large circle. If you roll a circle of radius $2$ twice around the circle of radius $3$ then the correspondence between points that meet isn't even a function in one direction, let alone a one-to-one function. I think perhaps you mean there is a mapping from some parameter to the pairs of points along the two circles that are brought into contact; for example, if the parameter is time in seconds since the motion started, the mapping says which two points will be in contact at each time $t$.

I prefer a completely different approach than the one in the video. Seat yourself on a frame that is attached to the centers of the two circles. As the small circle rolls around the larger one, the motion of the center of the small circle rotates the frame (and you) around the center of the larger circle.

While sitting on the frame, what you see is the two centers of the circles remaining in the same place within your field of view, while the larger circle rotates around its center and the smaller circle also rotates without slipping against the larger circle. And of course you see the smaller circle rotate three times.

But someone who remained motionless relative to the larger circle, not sitting in your rotating frame, sees the smaller circle rotate four times: the three times that you observed, plus one rotation that you did not observe because you were doing a full rotation in the same direction yourself. If you step off the frame and "undo" the effect of its rotation upon you by turning yourself once in the opposite direction, you'll observe the fourth rotation of the small circle.

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Given The radius of Circle A is 1/3 the radius of Circle B. Circle A rolls around Circle B one trip back to its starting point.

Question How many times will Circle A rotate about its center

*** Solution, Part 1 Begin by drawing (1) Circle B (2) Circle A where it initially contacts Circle B (3) Circle A where it contacts Circle B after rolling 1/3 the way around Circle B

Add to this drawing 3 lines Line 1 the line connecting the centers of Circles A and B when Circle A initially contacts Circle B

Line 2 the line connecting the centers of Circles A and B when Circle A has 1/3 the way around Circle B

Line 3 the line that is parallel to Line 1 and passes through the center of Circles A when Circle A has 1/3 the way around Circle B

*** Solution, Part 2 (1) The angle between Line 2 and Line 1 = 120 degrees, so (2) The angle between Line 2 and Line 3 = 120 degrees

So the total angle about Circle A's center through which the point of contact has rotated is 360 + 120 degrees

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There are many good explanations above, but I think that I can display a view a little bit different about this classic question:

Look at the below figure:

Circle rolling

When the blue circle rotates around the black circle the corresponding to the angle $\theta$, its center moves $\theta(R+r)$.

If one marks contact point P in the initial position, now it is in P’ and the new contact point is at Q. All translation movement in O is traduced in a rotation of P.

If one thinks in a tiny movement, the circle can be approximated by a polygon with thousands of sides.

if O move to O’ in a small segment part of a horizontal line, P rotates around O the same distance, as in the movement of a circle on a horizontal line.

So OO’ = PQ = QP’.

If $Q = 2\pi$ (complete turn), O moves $2\pi(R+r)$. as the blue circle has a perimeter of $2\pi r$, and assuming that $R = nr$, we get $2\pi (n+1)r$ that corresponds to n+1 complete laps

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Since 'revolution' has a mathematical definition, i.e. $1 \ rev = 2 \pi r $ radians, where $r$ is the radius, this problem is easily explained. We're looking for how many revs of Circle A occur.

Circle A:

Note: $r$ = radius(Circle B)

$1 \ rev = \frac{2}{3}\pi r $ radians

$2 \ revs = \frac{4}{3}\pi r $ radians

$3 \ revs = 2\pi r $ radians = Circle B's circumference which Circle A traversed.

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I don't know why people overthink these simple problems. The circumference of the larger circle is 3x the circumference of the smaller circle. Provided the small circle is not slipping, it will take 3 turns to cover the distance around the larger circle. Nothing magical here.

The reason you were perplexed by the video is because the uploader makes a mistake at 1:34 to 1:42. He counts this as a full revolution of the smaller circle, which it clearly is not. If he marked both circles with points on their circumferences, you'd see that it would take 3 revolutions. It's simply conservation of length. The rolling motion is completely irrelevant to the stated problem.

By the way, the applet you linked to was for a 4:1 scale factor, this is why in the applet it takes four revolutions.

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  • $\begingroup$ I think you dismiss the OP's self-answer to this two+ year old Question too glibly. While the small circle rotates three times with respect to the large circle, the OP and video identify an addition component of the small circle's motion (and conclude this contributes a fourth revolution). At a minimum you should address this thought in posting a new Answer so long after the original post was made. $\endgroup$ – hardmath Mar 23 '18 at 4:18
  • $\begingroup$ The video shows no such thing. At minimum you should address the point I made about the uploader mistaking 1:34 to 1:42 as a complete revolution. It is not a complete turn it merely realigns with the original compass setting. The fact you replied to my reply of a two year old post so quickly, tells me you are still interested for some reason. My post here was based on the fact I only saw the video mentioned yesterday, and one of the replies linked this website. I am free to answer any question at anytime. You are free to not read my replies also. $\endgroup$ – Hagbard Celine Mar 24 '18 at 6:32
  • $\begingroup$ This site exists to help students of math (at all levels). Discussion of YouTube videos is not central to the problem described in this Question despite your finding this from a link in a comment on the video. The Math.SE community is self-moderating, and my Comment above was prompted by seeing your post in a review queue. $\endgroup$ – hardmath Mar 24 '18 at 21:18
  • $\begingroup$ My friend. The initial question that started this whole discussion was based on the youtube video. The uploader of the video claimed that a maths question in the 1982 SAT examinations about the two circles had all incorrect answers. I was merely pointing out the error that the uploader made. The confusion was , I believe, based on his definition of the word "revolve". As a maths teacher I found the controversy surrounding this question bizarre. I always teach my students to cut through potential ambiguity by assuming the simplest form of a question. All the best. $\endgroup$ – Hagbard Celine Mar 25 '18 at 22:41
  • $\begingroup$ Per site standards a Question whose only "problem statement" is a link to a video would be closed (and if not ameliorated, deleted). Here the Question follows the link that motivated their post with a full problem statement (see the fixed width text, following "Problem: ..."). Answers should address that problem, hopefully in a definitive way using mathematical reasoning. Specifically it asks about an extra revolution of an outer circle rolling around the circumference of an inner circle. As a thought experiment I suggest shrinking the inner circle to a point and counting revolutions. $\endgroup$ – hardmath Mar 25 '18 at 23:02

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